1. Field of the Invention
This invention relates to a method for preventing the surface of a hydrogel of a water-soluble acrylamide-type polymer from being tacky.
2. Description of the Prior Art
Water-soluble polymers such as polyacrylamides, partially hydrolyzed products (anionic) of polyacrylamides or cationic or nonionic copolymers containing acrylamide (referred to hereinafter generically as an acrylamide-type polymer) have gained markedly increased acceptance in recent years as paper sizes, sheet-forming thickeners, water-treatment flocculating agents or enhanced oil recovery agents.
Known methods for producing acrylamide-type polymers include, for example, aqueous solution polymerization, suspension polymerization or precipitation polymerization. The aqueous solution polymerization method has been most widely used because it is economical and affords high-molecular-weight polymers easily. In order to conduct the aqueous solution polymerization method economically in the polymerization or drying step, it is desirable to maintain the concentration of starting monomer at a high level during polymerization.
However, when the monomer concentration during polymerization is gradually increased, the polymer becomes a highly viscous hydrogel, and handling of the polymer as a fluid becomes difficult. This would be an incentive to the development of a method for producing a powdery polymer which is easy to handle and which dissolves in water, by crushing the highly viscous gel, drying the gel with a rotary cylindrical dryer or a belt-type dryer, and then pulverizing the dried particles.
Because of the tackiness of the polymeric hydrogel being handled, its adhesion to other materials and to itself is very great, and this constitutes a marked disadvantage in the drying of the polymeric hydrogel. An attempt to solve this problem is disclosed in Japanese Patent Application (OPI) No. 135253/74 (corresponding to U.S. Pat. No. 3,905,122). The present invention is an improvement over this prior method.
Adhesion of a hydrogel (referred to hereinafter simply as a gel) of an acrylamide-type polymer to other materials and to itself is extremely great, and as a matter of course, a reduction of its adhesiveness is desirable even when the method of Japanese Patent Application (OPI) No. 135253/74 is to be performed.
Generally, in dealing with a highly adhesive material, the use of a container made of a material with good release properties is generally considered. Usually, fluorocarbon resins, of which polytetrafluoroethylene is typical, and polyolefins such as polyethylene or polypropylene have often been employed. The general practice is to use such a material with good release properties, and also to coat a suitable mold releasing agent on the material.
In order to accomplish this, various mold releasing agents have been developed, and typical examples of mold releasing agents are metal soaps, fatty acid amides, saturated hydrocarbons, and natural waxes. Certain silicon-containing compounds which are relatively stable chemically and do not affect the polymerization reaction may also be used as mold releasing agents.
Low adhesiveness of particles of the gel to each other is desirable to avoid a re-coalescence of small lumps of gel for drying, to prevent adhesion of small lumps of the gel during drying, and also to ensure the separation of the small lumps of the gel during or after drying into individual particles.
Good results would be obtained if the inside wall surface of the dryer is made of a non-tacky material. It is readily conceivable that the difficulties which are caused by the adhesion of gels to other materials could be avoided by constructing the surfaces of the other materials with a non-tacky material. Such a non-tacky material is usually a non-metallic material, and has low mechanical strength. Thus, the durability of such a non-tacky material in long-term operation under the severe conditions encountered in commercial operations is undesirably short. Another means for reducing the difficulties caused by the adhesion of gels to other materials is to use a mold releasing agent. Coating of the mold releasing agent on areas contacting the gel is effective for reducing the adhesiveness, and satisfactory results can be obtained in certain circumstances. However, the durability of mold releasing agents is shorter than that of non-tacky materials, and therefore, frequent coating of fresh mold releasing agent is required. During the re-coating operation, the operation should be stopped, and also the amount of labor involved in the re-coating operation is high.
The present invention offers a solution to these problems. It permits the adhesiveness of gels to be reduced, difficulties caused by the adhesion of gels to be eliminated even where the areas of the dryer which contact the gel are made of common metallic materials, and the adhesiveness of small gel particles to each other to be reduced and difficulties during drying to be removed.